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Use of solar energy for
preservation of agricultural products
By Dr Yasir Jamil and Muhammad Ramzan
Solar energy has been used for
time immemorial by man for his energy needs. The intensity
of use of solar energy also increased as human race
progressed through the initial ages when sun was put to use
not only for warmth, but also for productive applications.
Drying was perhaps the most important of such productive
applications. Drying of food grains, fruit and meat for
storage, drying of timber and firewood, drying of earthen
utensils, etc. were some such productive applications where
sun’s energy was put to effective use.
We can say that food drying is a very simple ancient skill.
It requires a safe place to spread the food where dry air in
large quantities can pass over and beside thin pieces. Sun
is often used to provide the hot dry air. Dry, clean air
including dry cold air from any source will dehydrate food.
Draping food over branches or spreading it on wide shallow
baskets on the roof is an old widespread tradition still in
use around the world. Many other arrangements have been used
to support a thin spread of food pieces. Some options that
have been used are to thread the pieces on a cord or a stick
and hang it over a fire, wood stove or from the rafter. Or
one can bundle herbs or strawflowers and suspend them from
bushes or a door knob or nails in rooms with good
ventilation.
But traditional sun drying methods often yield poor quality,
sine the product is not protected against dust, rain and
wind, or even against insects, birds, rodents and domestic
animals while drying. Soiling, contamination with
microorganism, of mycotoxins, and infection with
disease-causing germs are the result. They assert that the
drying equipment used in industrialized countries overcomes
all of these problems, but unfortunately is not very well
suited for use in developing countries because it requires
substantial investments and a well-developed infrastructure.
They further maintain that solar drying facilities combine
the advantages of traditional and industrial methods, namely
low investment costs and high products quality.
Dried foods are tasty, nutritious, lightweight, easy
–to-prepare, and easy –to-store and use. The energy input is
less than what is needed is needed to freeze or can, and the
storage space is minimal compared with that needed for
canning jars an freezer containers.
The nutritional value of food is only minimally affected by
drying. Vitamin ‘A’ is retained during drying, however,
because vitamin ‘A’ is light sensitive, food containing it
should be stored in dark places. Yellow and dark green
vegetables, such as peppers, carrots winter squash, and
sweet potatoes have high vitamin ‘A’ content. Vitamin ‘C’ is
destroyed by exposure to heat, although retreating foods
with lemon, orange, or pineapple juice increase vitamin ‘C’
contact. Dried foods are high in fibre and carbohydrates and
low in fat, making them healthy food choices. Dried foods
that are not completely dried are susceptible to mould.
Microorganism are effectively killed when the internal
temperature of food reaches 145 degrees Fahrenheit (F).
Dehydration of vegetables and other food crops by
traditional methods of open-air sun drying is not
satisfactory, because the products deteriorate rapidly.
Furthermore, traditional methods do not protect the products
from contamination by dirt, debris, insects, or germs. A
study demonstrates that food items dried in a solar dryer
were superior to those which were sun dried when evaluated
in terms of taste, colour, and mould counts. It asserts that
solar drying system must be developed to utilize this energy
resource to improve food preservation.
Drying is basically a heat and mass transfer process where
liquid water from the surface and form inside the grains is
vaporized, mixed with drying air and vapour mixture removed.
The drying air provides the heat. When drying foods, the key
is to remove moisture as quickly as possible at a
temperature that does not seriously affect the flavour,
texture and colour of the food. If the temperature is too
low in the beginning, microorganisms may grow before the
food is adequately dried. The food may harden on the
surface. If the temperature is too high and the humidity is
too low, it makes it more difficult for moisture to escape
and the food does not dry properly. Although drying is a
relatively simple method of food preservation, the procedure
is not exact.
BIn industries, dryers are used for drying of
powders, seeds, cloth, paper, yarn and a large
number of other products. The basic objective of
drying operations in industries is to remove the
moisture which has come into product during the
process of manufacture. This is necessary to make
the product suitable for marketing and also for
increasing the shelf life. A majority of industrial
dryers use steam/electricity for hearting operation
required for removal of moisture.
Some dryers also use fuels like kerosene and diesel
for hearting the air which is to remove the
moisture. Most of these dryers work at temperatures
ranging between 60 and 70 C. Many dryers temperature
limits are specified at even lower value so that
products which are heat sensitive are not spoiled
because of over heating. It is also known that most
of these dryers work at efficiencies between 30 and
6 per cent in comparison to even low efficiency of
open sun drying. This shows the enormous, potential
available for saving of conventional fuels be
conversion, wherever possible, to solar energy based
drying operations.
Use of solar energy for drying in industries can
also offer advantages like better quality of the
product, less pollution and freedom from unreliable
supply of oil, electricity and coal. In fact in many
small scale industries it has been seen that quick
drying is the bottle-neck operation in increasing
the productivity. These industries have then to
resort to open sun drying of their product which not
only requires space but also increases drying time
and reduces the productivity.
Drying preserves foods by removing enough moisture
from food to prevent decay and spoilage. Water
content of properly dried food varies from 5 to 25
percent depending on the food. Successful drying
depends on:
(i) Enough heat to draw out moisture, without
cooking the food;
(ii) Dry air to absorb the released moisture; and
(iii) Adequate air circulation to carry off the
moisture.
Now we look in agriculture in our country where a
substantial quantity of our country’s grain and
fruit production is wasted because of inadequate and
improper storage facilities. Growth of insects and
fungus which are the two main reasons for spoilage
of these materials can be prevented to a large
extent by ensuring reduction of moisture contents to
safe limits before storage. Extensive research has
been carried out on the moisture requirements of
various types of food material before they can be
stored for long term. Table shows the recommended
values of final moisture content of various food
materials for long term and safe storage. (See
Table-1)
The drying of food material is primarily a low
temperature operation as higher temperatures are
likely to result in destruction of nutrients and
flavour of the food. Recommended values of drying
temperatures for various food materials are given in
the table. A study of these temperatures reveals the
reason why solar energy is being considered as
appropriate sources of energy for drying.
It is well known that solar energy collection is
most efficient at temperature up to 70 C. While use
of conventional fuels at low temperature ranges is a
most inefficient way of using these high grade
energy sources. Various types of dryers have been
developed at a number of institutes for use,
especially for agricultural products. These dryers
have been used for drying of seeds, paddy, various
types of fruits, chillies and a variety of other
vegetable products. The capacities of these dryers
range from a few kg/day to a few tones/day. (See
Table-2)
Various agricultural products require reduction in
their moisture contents for storage purposes.
Moisture contents of Paddy are oil seeds, corn etc.
are reduced for safe storage. Dehydration of
vegetables and fruits is a cheap method of storing
these products for off-season method use. Similarly;
tea leaves, tobacco leaves and coffee also require
drying.
Traditionally solar energy has been used to dry
agricultural products by spreading them on the
ground in open sun. This method of open sun drying
is very time consuming. Also the product gets
contaminated by dirt and insects. The use of solar
dryers overcomes these problems to large extent.
Solar dryers make use of solar heated air for drying
agricultural products. Solar dryers developed so far
are of two types;
(1) Forced convective solar dryer
The solar dryer consists of solar air heater of flat
plat type and a drying chamber. The air heater forms
the roof and drying chamber is placed in an
enclosure formed under the air heater. The air
heater is connected to the drying chamber through an
air blower. The blower sucks air through the air
heater and blows it through the drying chamber. A
schematic of the system is showing figure. The solar
air heater (flat plate type) has air flow depth of
2.5 cm.
It consists of four sheets in parallel with total
area of 28m2. the drying chamber has been designed
for manual loading and unloading of product to be
dried. Depth of grains can be up to 40cm.
(2) Natural convective solar dryer
Natural convective solar dryer consists on three
different types
* Cabinet dryer
* Multi-rack dryer
* Mini multi-rack dryer
Cabinet dryer
It is a singe rack natural convective dryer. It is a
wooden hot box with glass at the top. The produce to
be dried is placed on a perforated tray. Holes are
provided for natural convective flow of air.
Multi-rack dryer
In the cabinet dryer the dry air comes in contact
with the product only once. This air has still
capacity to remove more moisture if it could be
passed through additional layers of product.
Mini Multi-rack dryer
A small sized multi-rack dryer in which 5 trays at 5
different levels in vertical direction can be
placed, may be designed for house–hold use.
These solar dryers are very low cost even a middle
class farmer can also made them at a low cost few
hundred rupees and can use them for long years. Use
of solar dryers for agriculture purpose in
developing countries like Pakistan has a wide scope
as water for irrigation is available for us for a
few days on counting system and we have a limited
time to save our crops after harvesting and to
sowing next crops. This time shortage problem can be
overcome by using low cost solar dryers because they
dried the crops, which need 7 days for drying, in 7
hours.
Another factor is very important to remember (which
is one of major reasons to use solar dryers) is our
regional weather, which is severe and not reliable
and destroys a substantial our food, vegetables and
crops. If you want to become a modern cultivator and
want to earn a reasonable capital by saving y our
time and crops, then these dryers are not out of
reach.
TABLE-1: INITIAL M.C. & FINAL M.C. OF VARIOUS
PRODUCTS
Harvesting corn/Fruits/vegetables etc.
Initial M.C. Final M.C. after
dehydration
(%) (%)
Cereals & grains 16-30
10-12
Pulses 20-22
9-10
Oilseeds
12-14
7-9
Vegetables
60-80
10-12
Fruits 60-90
10-12
Tea leaves
50
8
Coffee beans
50
12
Note: M.C. stands for moisture contents
TABLE-2: OPTIMUM TEMPERATURE RANGE
FOR DEHYDRATION OF FRUITS AND VEGETABLES
Vegetables
Optimum Fruits
Optimum
Temperature (co)
Temperature (Co)
Bitter Gourd
65-70
Apples
60-65
Bringer 50-55
Apricot
55-60
Cabbage
60-65
Bananas
50-55
Carrot 65-70
Grapes
65-85
Cauliflower
60-65
Peaches
60-70
Garlic 55-60
Pears
60-65
Okra (lady finger) 65-70
Beers
60-80
Onion 60-65
Pomegranates 50-60
Peas 60-65
Figs
50-60
Potatoes
60-65
Dates
60-80
Pumpkin
65-70
Spinach
65-70
Tomatoes
60-65
Turnip 50-55
Groundnut
40-45
Seed purpose – should not be more than 40oC.
Chillies 40-45
Flower perfume 45oC
Tobacco
30-35
Tea 30-35
Courtesy: The NEWS
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Pakissan.com;
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